The scientific and operational aims of the Hard X-Ray Spectrometer (HXRS) are discussed. The principal operating characteristics of the instrument such as the temporal resolution, energy band selection, spectral sensitivity, and the in-night calibration procedure are described as well as the technical details of the spectrometer including : detectors, shielding (against charged particles) and electronic design. The Multispectral Thermal Imager (MTI) host satellite, and orbit are briefly described.